In general, two categories of biological processes are available for wastewater treatment: aerobic processes and anaerobic processes. Aerobic processes utilize bacteria which require oxygen to convert organic materials primarily to water and carbon dioxide. Anaerobic processes, on the other hand, utilize bacteria which grow in the absence of oxygen and convert organic materials primarily to the end products methane and carbon dioxide. It has been found that for high strength wastewaters, that is wastewaters having a Chemical Oxygen Demand (COD) greater than 2000 mg/L, anaerobic processes are more advantageous because (1) they require less energy and (2) they produce between one-tenth and one-fifth of the residual biomass resulting from aerobic processes.
With respect to the treatment of high strength wastewaters having a Total Suspended Solid (TSS) level greater than 500 mg/L, two categories of anaerobic processes are effective. The first anaerobic process category utilizes anaerobic lagoons which exist as large pits or vessels. With anaerobic lagoons, wastewater is simply introduced into one end of the lagoon whereby suspended anaerobic bacteria within the lagoon degrade both dissolved and particulate organic materials. After an average retention time of several weeks, treated wastewater flows from the anaerobic lagoon.
A second anaerobic process category for wastewater treatment is anaerobic contact processing. Anaerobic contact processes utilize a flow-through, closed top reacting vessel containing anaerobic bacteria. Wastewater flowing into the reacting vessel interacts with the anaerobic bacteria forming an anaerobic liquor. The anaerobic liquor subsequently flows to a settling vessel or clarifier wherein the bacterial solids settle to the bottom of the settling vessel, leaving the relatively clean wastewater to overflow from the top of the settling vessel. Settled solids are then pumped back to the reactor and the process is continued. Before entering the clarifier, the anaerobic liquor passes through a degassifier to minimize the occurrence of super-saturated gases. A typical problem with the anaerobic contact process, however, is achieving good gravity separation of the bacteria from the anaerobic liquor under atmospheric conditions so that only relatively clean wastewater is decanted from the settling vessel.
Several embodiments exist for applying anaerobic processes, a more recent embodiment is the anaerobic sequencing batch reactor (ASBR). An ASBR operates by partially filling a reactor vessel containing anaerobic bacteria with wastewater and gently mixing the contents so to assist the anaerobic bacteria in degrading the organic materials of the wastewater. After the anaerobic bacteria react with the wastewater, the mixer is turned off, allowing the anaerobic bacteria to settle to the bottom of the reactor vessel. The treated wastewater is then decanted from the top of the reactor vessel. Limitations, however, of this anaerobic process are (1) the need for a relatively large feed equilization to accommodate the batch operation and 2) the need to provide a variable-level tank, decanter, and gas collection system.
For wastewaters having a TSS greater than 500 mg/L, lagoons and anaerobic contact processing systems are more effective in treatment. Lagoons, however, require large land space while anaerobic contact systems are subject to settling problems resulting from inadequate degassification or even regassification in the clarifier. An anaerobic contact system is also costly because it requires multiple components: i.e., a separate equalization reactor, a degassifier, outdoor clarifier vessels, associated return pumps, and piping.
The inventor of the invention described herein has developed an apparatus which modifies the conventional application of the anaerobic contact process. Use of this invention precludes the degassification problems occurring in the clarifier device associated with the conventional anaerobic contact process without requiring batch operation or variable-level operation. Additionally, the invention described herein can be manufactured and used at a cost which is much less than that required for an apparatus applying the conventional anaerobic contact process.